1. Field of the Invention
The present invention relates to a zoom lens system for a single lens reflex camera (a SLR camera), and in particular, a zoom lens system suitable for a digital SLR camera.
2. Description of the Related Art
A digital SLR camera has used an imaging device having an imaging surface which is smaller than the imaging surface of a SLR camera with a silver halide film (hereinafter, a silver-halide-film SLR camera). This is because it has been difficult to achieve a larger imaging device.
Accordingly, in the case where an exchangeable photographing lens system with a predetermined focal length is used with a digital SLR camera, the angle-of-view thereof becomes narrower than the case where the same exchangeable photographing lens system is used with a silver-halide-film SLR camera. Therefore an exchangeable photographing lens system having a shorter focal length is desired for a digital SLR camera.
On the other hand, the back focal distance of a digital SLR camera is required to be longer. This is because the lens mount and the flange back have been made common to both digital and silver-halide-film SLR cameras so that the various types of exchangeable photographing lens systems for silver-halide-film SLR cameras on the market can be utilized for digital SLR cameras.
In order to satisfy the above requirements for a digital SLR camera, it is advantageous to employ a negative-lead type zoom lens system of a four-lens-group arrangement, i.e., a first lens group having a negative refractive power (hereinafter, a negative first lens group), a second lens group having a positive refractive power (hereinafter, a positive second lens group), a third lens group having a negative refractive power (hereinafter, a negative third lens group) and a fourth lens group having a positive refractive power (hereinafter, a positive fourth lens group), in this order from the object.
However, in a conventional negative-lead type zoom lens system of a four-lens-group arrangement, the structure of the negative third lens group and that of the positive fourth lens group have been complicated; and the production costs thereof have increased. Furthermore, in the above conventional negative-lead type zoom lens system, distribution of refractive power over each lens group has not been sufficiently considered, so that the amount of peripheral illumination tends to be decreased.